The invention relates to a heatable electrode for igniting and reigniting high-pressure gas discharge lamps; and more particularly to such an electrode which comprises a self-supporting cylindrical open coil of tungsten or molybdenum wire having a metal wire of tungsten or molybdenum passed coaxially through the coil and connected thereto. Such electrodes serve to reduce the ignition voltage of gas discharge lamps; the heating of the electrode can either be maintained during the whole operation of the lamp or be switched off after ignition of the lamp.
In an electrode of this kind for the hot reignition of 400 W mercury vapor high-pressure lamps known from DE OS No. 3106201, which corresponds to U.S. Pat. No. 4,398,123 the coil consists of a comparatively thick wire and its individual turns are pulled far apart so that they are not in contact with each other. At the outermost turn, the coil wire passes without variation in cross-section into the metal wire passed coaxially through the coil. For preheating, the whole electrode is heated uniformly. This requires comparatively high currents for the thick coil wire, which currents can be allowed to flow through the seals of the lamp ends in high-power lamps only.
U.S. Pat. No. 3,519,872 discloses a high-pressure gas discharge lamp having an electrode which is cylindrically wound onto a tungsten rod, and has an outer end which is connected a heating coil consisting of thin wire. When an ignition pulse is applied to the lamp after this electrode has been preheated, the discharge arc first starts at the heating coil and then travels to the actual cylindrically wound electrode. This requires a geometry of the electrode arrangement that has to be accurately maintained. Moreover, there is a risk of the discharge arc remaining on the thin heating coil for too long a time and thus leading to its destruction.
The known preheatable electrodes are in addition not suitable for so-called miniaturized gas discharge lamps, i.e. gas discharge lamps consuming a power of up to about 70 W, because of the comparatively large electrode dimensions.